Prop slip

[blh1972]

What is the accepted % of slip for a prop?
when I plug my numbers into prop calculator's it shows about 30% slip.

 

[Don S]

Re: Prop slip

Moving this to the prop forum.

 

[blh1972]

Re: Prop slip

Nobody???

 

[rogerwa]

Re: Prop slip

Not an expert here but it depends on what you are pushing. A displacement hull will have a larger slippage. I have a 25' pontoon. The 40hp motor prop gave a 40+%. When I upgraded to a 90hp, with a larger diameter it brought the slippage down to around 29%.

Again it depends on what your setup is.

 

[blh1972]

Re: Prop slip

It's a 26' searay. 5400 lbs.

 

[rogerwa]

Re: Prop slip

What are the other specs.. WOT, speed, prop size and pitch.. Etc?

 

[blh1972]

Re: Prop slip

14 3/4 x 15 stainless prop, 4800rpm, 29-31 mph (by gps).
gear ratio is 1.5 to 1

 

[bekosh]

Re: Prop slip

I use this - http://www.go-fast.com/Prop_Slip_Calculator.htm
With your numbers, I get a slip of 33% which seems high to me.
When I use my numbers,
Drive Ratio 1.5 to 1, Mirage Plus 14.625x21 SS prop, 4800rpm, GPS speed 57.5mph, I get 10% slippage.

From the go-fast page it says this about slippage.

Slip is never a negative number, and will range from 5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent.

 

[MikDee]

Re: Prop slip

If you have a 350 Vortec in there as in your signature, I think you might benefit from a 17" pitch prop, it will drop your rpm down I'd say into the recommended 4200-4600rpm range, and pick up some speed, with less slip.

 

[rogerwa]

Re: Prop slip

Again not an expert here. You may try different props with a larger diameter (more blade surface) or cup (better bite) or both to see if it impacts your numbers. If you look at the prop finder here on Iboats you see a number of props for that application that are 15"+. Also without knowing what prop is on there, its hard to know if it is cupped or not.

Experimenting would definitely be worth the trouble since you could gain 6-7 mph if you were to bring it to 15%.

Also, is the hull weight the listed dry weight or is this measured with fuel, gear, water, etc. You may running well in excess of 5500lbs.

 

[QC]

Re: Prop slip

You may try different props with a larger diameter (more blade surface) or cup (better bite) or both to see if it impacts your numbers. If you look at the prop finder here on Iboats you see a number of props for that application that are 15"+

This is why many heavier cruisers are set up with Bravo 2s. In this case, diameter is called for . . .

 

[blh1972]

Re: Prop slip

So I should try a larger diameter prop in the same pitch?
How big can I go with the alpha one drive?

 

[rogerwa]

Re: Prop slip

I would stick with the 15P since you are at max WOT. The increased diameter may even lower your RPM.

Here are the Iboats options for the 15"p
http://www.iboats.com/mall/propfind...id=443&**********=835338060&*******=159637168

 

[blh1972]

Re: Prop slip

Thanks guys for the responses.
I will try a larger diameter inthe same pitch.
Does anyone know the max diameter for an alpha 1 outdrive.

 

[rogerwa]

Re: Prop slip

I don't know the max, but if you browse the prop for your application there are some 16" diameters in there. Maybe they sacrifice some of the exhaust for diameter.

You may be even able to handle going up an inch in pitch. One inch theoretically would drop your RPM by 200 but give you more speed. I just don't know how the incresed diameter will impact RPM.

 

[MikDee]

Re: Prop slip

I don't know the max, but if you browse the prop for your application there are some 16" diameters in there. Maybe they sacrifice some of the exhaust for diameter.

You may be even able to handle going up an inch in pitch. One inch theoretically would drop your RPM by 200 but give you more speed. I just don't know how the incresed diameter will impact RPM.

Thinking about it, it does seem logical to inrease diameter, but it will drop rpm, so now I wouldn't recommend an increase in pitch. I've had good luck with Mercruiser props, over Michigans, but I know they're more expensive.